/*
 * @Author: your name
 * @Date: 2021-10-03 19:40:39
 * @LastEditTime: 2021-10-03 20:10:56
 * @LastEditors: Please set LastEditors
 * @Description: In User Settings Edit
 * @FilePath: \luogu\1.6二分查找\P1163\main2.cpp
 */
#include<iostream>
#include<iomanip>
using namespace std;

//本金 期数 期款 期利率 计算到期后欠银行的钱数
double getEndCount(double a, double n, double d, double i);
//二分
double binarySearch(double a, double n, double d);

int main(){
    //a:本金 n:期款 d:期数
    double a,d,n;
    cin >> a >> d >> n;
    cout << fixed << setprecision(1) << binarySearch(a,n,d)*100 << endl;
    // cout << setprecision(3) << binarySearch(a,n,d) << endl;
}
double getEndCount(double a, double n, double d, double i)
{
    double count = a;
    //第i个月过完欠银行的前
    for(int j=1; j<=n; j++){
        //余额 = 本金+利息-还款
        count = count + count*i - d;
    }
    //最后欠银行的钱
    return count;
}
double binarySearch(double a, double n, double d)
{
    double low = 0;
    double high = 5;
    double mid;
    while(low < high){
        mid = (low+high)/2;
        double remain = getEndCount(a,n,d,mid);
        //提前退出判断
        if(remain <= 0.0001 && remain >= -0.0001) return mid;
        //银行欠我钱,预估利息过低
        if(remain < 0){
            low = mid+0.0000001;
        }else{
            //我依旧欠钱,预估利息过高
            high = mid-0.0000001;
        }
    }
    return high;
}